I recently stumbled on an issue with Dropbox and the Ubuntu flavors that support AppIndicators (except Unity), like Xubuntu and Lubuntu: the Dropbox AppIndicator icon shows up as broken and the menu doesn't work. This isn't a new issue though, and it seems to occur starting with Ubuntu 14.04.

The issue occurs with the Dropbox packages in the official Ubuntu repositories (called "nautilus-dropbox", which doesn't depend on Nautilus and can be used to install Dropbox on any desktop environment) as well as the Dropbox package downloaded from its official website.

It does not occur with the caja-dropbox package available in the official Ubuntu MATE 16.04 repository though (but it does occur in older Ubuntu MATE versions if you've enabled AppIndicators), because it was patched with a fix similar to the one in this article.

Below you'll find a fix / workaround for this issue. Important: using the instructions below, Dropbox will use a tray (notification area) icon instead of an AppIndicator.

Here's a screenshot with the issue (taken in Xubuntu 16.04):

And another screenshot taken after using the fix below:

Tested in Xubuntu 16.04, Xubuntu 14.04, Lubuntu 16.04, Ubuntu MATE 16.04 (the issue does not occur with the caja-dropbox package in this Ubuntu MATE version) and Ubuntu MATE 14.04.

To fix it, you need to add "dbus-launch" before the actual command for the "Exec" line in both the application autostart file and launcher. For Dropbox this is a bit tricky because it overwrites any modifications to its autostart file. So here's what you need to do to fix this Dropbox issue:

if you've installed Dropbox by downloading the .deb from its website or by using the nautilus-dropbox package from the repositories:

rename the Dropbox autostart file, located in ~/.config/autostart/, and edit the file, changing the "Exec" line to "Exec=dbus-launch dropbox start -i";

copy the Dropbox desktop file, located under /usr/share/applications/, to ~/.local/share/applications/, so it's not overwritten when updating the package, and change the "Exec" line to "Exec=dbus-launch dropbox start -i"

for the caja-dropbox package (except for Ubuntu MATE 16.04 which doesn't have this issue):

rename the dropbox-caja autostart file, located in ~/.config/autostart/, and edit the file, changing the "Exec" line to "Exec=dbus-launch caja-dropbox start -i";

copy the caja-dropbox desktop file, located under /usr/share/applications/, to ~/.local/share/applications/, so it's not overwritten when updating the package, and change the "Exec" line to "Exec=dbus-launch caja-dropbox start -i";

This sounds a bit complicated on a first look, right? Well, it's not, but to make it easier, you can use the following commands to apply the changes I mentioned above.

If you've installed Dropbox by downloading the .deb from its website or by using the nautilus-dropbox package, you can fix the broken Dropbox appindicator icon and menu by using the following commands:

Then restart the session (logout/login) and the Dropbox icon and menu should work correctly.

Update: the solution above also works for the GNOME Flashback session, but with a tweak. You'll need to add the "Notification Area" applet to the panel. To add applets to the panel, you must hold the ALT key while right clicking the panel, then select "Add to panel". In some cases, you must hold down both the ALT and the Super (Windows) keys while right clicking the panel so try this if just holding ALT doesn't work.

Update 2: Dropbox has ended support for some filesystems which its devs deem as uncommon, so it only works on Linux with Ext4 now. Users running other filesystem types, be it Ext3, Zfs, Btrfs, Xfs, and even an encrypted Ext4 filesystem can no longer synchronize their files with the Dropbox Linux application. However, there is a way to get dropbox to support any filesystem type on Linux, which you can read HERE.